The increased demand in recent years for off-road and all terrain vehicles has led to tremendous developments in those types of vehicles. Many of the developments have centered around making the vehicle more adaptable to changing road conditions, e.g., dirt roads, pavement and gravel. As the road terrain changes, it is desirable to vary the driving capabilities of the vehicle to more efficiently navigate the new terrain. Prior four-wheel drive and all terrain vehicles were cumbersome since they required the operator to manually engage and disengage the secondary drive shaft, e.g., by stopping the vehicle to physically lock/unlock the wheel hubs. Improvements in vehicle drive trains, such as the development of automated systems for engaging and disengaging a driven axle, eliminated many of the problems of the prior designs. These automated drive systems are sometimes referred to as “on-the-fly” four wheel drive. These systems, however, require the vehicle to be in either 2-wheel or 4-wheel drive at all times.
Generally, all four-wheel drive vehicles include a differential for transferring torque from a drive shaft to the driven shafts that are attached to the Wheels. Typically, the driven shafts (or half shafts) are independent of one another allowing differential action to occur when one wheel attempts to rotate at a different speed than the other, for example when the vehicle turns. The differential action also eliminates tire scrubbing, reduces transmission loads and reduces understeering during cornering (the tendency to go straight in a corner). There are four main types of conventional differentials: open, limited slip, locking, and center differentials. An open differential allows differential action between the half shafts but, when one wheel loses traction, all available torque is transferred to the wheel without traction resulting in the vehicle stopping.
A limited slip differential overcomes the problems with the open differential by transferring all torque to the wheel that is not slipping. Some of the more expensive limited slip differentials use sensors and hydraulic pressure to actuate the clutch packs locking the two half shafts together. The benefits of these hydraulic for viscous) units are often overshadowed by their cost, since they require expensive fluids and complex pumping systems. The heat generated in these systems, especially when used for prolonged periods of time may also require the addition of an auxiliary fluid cooling source.
The third type of differential is a locking differential that uses clutches to lock the two half shafts together or incorporates a mechanical link connecting the two shafts. In these types of differentials, both wheels can transmit torque regardless of traction. The primary drawback to these types of differentials is that the two half shafts are no longer independent of each other. As such, the half shafts are either locked or unlocked to one another. This can result in problems during turning where the outside wheel tries to rotate faster than the inside wheel. Since the half shafts are locked together, one wheel must scrub. Another problem that occurs in locking differentials is twitchiness when cornering due to the inability of the two shafts to turn at different speeds.
The final type of differential is a center differential. These types of differentials are used in the transfer case of a four wheel drive vehicle to develop a torque split between the front and rear drive shafts.
Many differentials on the market today use some form of an overrunning clutch to transmit torque when needed to a driven shaft. One successful use of an overrunning clutch in an all terrain vehicle is disclosed in U.S. Pat. No. RE38,012, commonly owned by the assignee of the present invention, describes a bi-directional overrunning clutch for controlling torque transmission between a secondary drive shaft and secondary driven shafts. This transmission system is beneficial in four wheel drive vehicles where it is desirable to be able to engage and disengage the secondary driven shafts in different driving environments. The system described in U.S. Pat. No. RE38,012 includes an innovative system to advance and/or retard a roll cage, thereby controlling the ability of the differential to engage and disengage depending on the operational state of the primary and secondary wheels. The system includes an electromechanical device, which in one embodiment is an electrically controlled coil adjacent to an armature plate that is engaged with the roll cage and rotates in conjunction with the roll cage. When the coil is energized, an electromagnetic field is produced which hinders the rotation of the armature plate, thus causing the roll cage to drag or advance into an appropriate position relative a clutch housing.
The differential in U.S. Pat. No. RE38,012 also includes a backdrive system that actively engages the secondary shafts in certain situations where extra traction is needed. For example, when the vehicle is driving down a slope and the differential is used in the front differential, the system engages the front wheels, which are the wheels with the better traction.
U.S. Pat. No. 6,622,837, commonly assigned to the assignee of the present invention, describes a differential system that includes a bi-directional overrunning clutch with automatic backdrive capability. In this system, an electromagnetic device is used to engage an armature plate that is keyed into the roll cage through tangs. Energizing of the electromagnetic device attracts the armature plate causing it to drag the roll cage, thereby placing the clutch in the activated position for four wheel drive. In another embodiment, a hydraulic piston engages the roll cage causing it to drag.
U.S. Pat. No. 6,722,484 discloses another bi-directional overrunning clutch that is useful on the primary drive axle for providing continuous engagement with overrunning capability, while at the same time providing engine braking capability. The overrunning clutch includes at least one friction member which is in contact with the roll cage and the hub such that, during operation, the friction member generates friction forces between the roll cage and the hub which cause the roll cage to turn with the hub, thus placing the roll cage in the forward-engagement position.
In the systems described above, the overrunning clutch was incorporated into the differential between the ring gear and the two secondary driven shafts so as to independently control engagement and disengagement of those driven shafts by directly engaging/disengaging the hubs of the secondary driven shafts. The location of the clutch at the driven shaft in the prior systems means that the clutch components have to be designed heavier in order to withstand high torque. As engines, transmissions, and vehicles evolve; the amount of torque required to be driven by a differential is increasing. In turn, this causes current overrunning clutches to get bigger, taking up space that may not be available. A need still exists for an improved differential system that can transmit higher torque in a smaller package.